Men only shop for clothes for two reasons.
Ever.
Reason #1: Money
Men will shop for clothes if it involves their jobs, an interview, or when they've got to do something that involves them ruining a pair of clothes. If he is unemployed, and rule #2 does not apply, he will continue to wear the same hole-ridden undies for weeks on end, until they fall apart. At this point, he will find a towel to wrap around his waist, or, alternately, walk around in the nude, hoping some Mormon missionaries come to the door.
Reason #2: When it's about a girl
Men will spend much more time (and money) shopping for clothes, when it's about a girl (or another man, depending on sexual persuasion).
Men will actually care about the fit, the color, the overall look of a piece of clothing when it involves a girl.
It doesn't even have to be a girl they like. If, for example, a man is going to a party, and finds out that his ex-girlfriend may show up, he'll go clothes shopping to show her that he is still a sharply dressed individual, who has not yet resorted to walking around in a towel wrapped around his waste (see rule number 1).
Furthermore, this is a good gauge for a woman to tell if a man wants to take the relationship up another level. If he asks her to go clothes shopping with him, or asks for an opinion on some clothing, then he is definitely ready for a heavier commitment.